DIFFERENT ACTIVATION-ENERGIES IN GLUCOSE-UPTAKE IN SACCHAROMYCES-CEREVISIAE DFY1 SUGGEST 2 TRANSPORT-SYSTEMS

The analysis of initial glucose uptake in Saccharomyces cerevisiae at 25 degrees, 20 degrees, 15 degrees and 10 degrees C by computer-assist ed nonlinear regression analysis predicts two transport systems. The f irst demonstrates Michaelis-Menten kinetics and the second shows first order behaviour. The activation energies of these two systems were ca lculated by the Arrhenius equation at four different growth phases, na mely early exponential (EE), middle exponential (ME(2)), late exponent ial (LE) and early stationary (ES) with 2% glucose in the batch medium . The activation energies calculated from the V-m values in EE, ME, LE and ES growth phases were 15.8 +/- 1.7, 13.5 +/- 1.0, 15.1 +/- 0.8 an d 13.5 +/- 0.7 kcal/mol. These values are in agreement with activation energies calculated for the first mechanism, facilitated diffusion, w hich is the mechanism deduced from countertransport experiments. The a ctivation energies derived for the second transport system from the fi rst order rate constants in cells grown to EE, ME(2), LE and ES were 8 .0 +/- 2.1, 8.1 +/- 1.3, 9.6 +/- 3.0 and 7.5 +/- 2.6 kcal/mol. These v alues are still significantly higher than for free diffusion of glucos e in water and lower as predicted for passage of glucose through the l ipid phase. Therefore, we assume in addition to carrier-mediated facil itated diffusion the entrance of glucose into the cell through a pore.